1. Field of the Invention
The present invention relates to an apparatus and a method for executing bias temperature (BT) treatment on semiconductor wafers.
2. Description of the Related Art
In a wafer process wherein movable ions, such as sodium ions (Na.sup.+), are added into an insulating film formed on the surface of a semiconductor wafer gradual movement of the movable ions by an electric field lowers stability at the surface of the semiconductor wafer.
The quantity of movable ions in the insulating film is generally determined by BT treatment and C-V (capacity-voltage) measurement.
Conventional BT treatment is executed by applying a d.c. (direct current) bias to an electrode formed on part of an insulating film of a semiconductor wafer under high temperature condition.
FIG. 1 shows a method of conventional BT treatment, where an electrode 201 is formed on an insulating film 102 of a semiconductor of wafer 100 through vapor deposition of a metal like aluminum (AL). A d.c. power source 301 applies a d.c. bias between the electrode 201 and a semiconductor substrate 101 of the semiconductor wafer 100. The d.c. bias is while the semiconductor wafer 100 is at a high temperature to allow movement of movable ions that are present in the insulating film 102. When the temperature of the semiconductor wafer 100 decreases, the movable ions moving in the insulating film 102 are bound to their new positions. Conventional C-V measurement is then performed by applying an a.c. (alternating current) voltage between the electrode 201 and the semiconductor substrate 101 of the BT-treated semiconductor wafer 100.
The quantity of movable ions is determined in the following manner through the conventional BT treatment and C-V measurement.
FIGS. 3(a), 3(b), and 3(c); show movement of sodium ions (movable ions) under the condition of BT treatment according to the method of FIG. 1.
BT treatment is executed while the positive terminal of the d.c. power source 301 is connected to the electrode 201 and the negative terminal of source 301 is connected to the semiconductor substrate 101 as shown in FIG. 1. Sodium ions existing at random positions in the insulating film 102 as shown in FIG. 3(a) start moving in the insulating film 102 to be attracted to the negative side, that is, toward the semiconductor substrate 101 as shown in FIG. 3(b). Such BT treatment is hereinafter referred to as `+BT treatment`.
When the temperature of the semiconductor wafer 100 decreases, the sodium ions are bound to their positions shown in FIG. 2(b). C-V measurement is then executed under the condition of FIG. 3(b).
BT treatment is then executed while the positive and negative terminals of the d.c. power source 301 are connected to the semiconductor substrate 101 and the electrode 201, respectively, contrary to the case shown in FIG. 1. The sodium ions bound to the positions shown in FIG. 3(b) start moving again in the insulating film 102 to be attracted to the negative side, that is, toward the electrode 201 as shown in FIG. 3(c). Such BT treatment is hereinafter referred to as `-BT treatment`.
As the temperature of the semiconductor wafer 100 decreases, the sodium ions are bound to their positions shown in FIG. 3(c). C-V measurement is then executed under the condition of FIG. 3(c).
FIG. 2 is a characteristic chart showing results of C-V measurement on the semiconductor wafer after +BT treatment and -BT treatment.
A C-V curve obtained by C-V measurement shifts from an ideal C-V curve along the voltage axis due to various causes. One of the causes is presence of movable ions, such as, sodium ions, in the insulating film.
A shift of flatband voltage due to movable ions in the insulating film is obtained as below. When movable ions (sodium ions) have charge density .rho.(x) in the insulating film 102 at a position x, where x is a distance from a boundary between the electrode 201 and the insulating film 102 as shown in FIG. 3(a), the shift of flatband voltage is given as an integral of a function x.rho.(x), that is, -x.rho.(x) dx.
under the condition of -BT treatment, all the movable ions (sodium ions) are positioned close to the electrode 201 as shown in FIG. 3(c). Therefore the drift of flatband voltage obtained as a result of integration becomes equal to zero. The C-V curve under the condition of -BT treatment is given as a curve Q shown in FIG. 2.
Under the condition of +BT treatment, on the other hand, all the movable ions (sodium ions) are positioned close to the semiconductor substrate 101 as shown in FIG. 3(b). Therefore the shift of flatband voltage is substantially equal to t.rho.(t), where t is a thickness of the insulating film 102. Since the thickness t of the insulating film 102 is a known value, the charge density .rho.(t) is obtained from the shift of flatband voltage t.rho.(t). Further the quantity of movable ions can be calculated from the charge density .rho.(t). The C-V curve under the condition of +BT treatment is given as a curve P shown in FIG. 2.
Since the shift of flatband voltage under the condition of -BT treatment is equal to zero as described above, the shift of flatband voltage from an ideal C-V curve under the condition of +BT treatment is obtained as a difference .DELTA.Vfb between the C-V curve Q in -BT treatment and the C-V curve P in +BT treatment shown in FIG. 2.
As described previously, the conventional method of BT treatment uses a semiconductor wafer having an electrode formed on part of an insulating film thereof, thus requiring a time-consuming and labor-consuming process of forming the electrode on the semiconductor wafer through vapor deposition of a metal film, prior to BT treatment.
The conventional method of BT treatment executes -BT treatment after +BT treatment (or +BT treatment after -BT treatment), thus taking a relatively long time.